Autism is a developmental disorder defined by deficits in reciprocal social interaction and communication; and by restricted, stereotyped interests and behaviors. Although autism is most typically diagnosed at age 4-5 years, many parents recognizing something unusual about their child's development by the second year of life, and most seek help by the time their child is 3. However, it is not clear whether autism can be diagnosed reliably in very young children, and if so, on what bases. The purpose of this project to investigate whether or not valid and reliable diagnoses of autism can be made of children ages 3 years and under according to research diagnostic criteria (DSMIII-R); APA 1987; ICD-10 Draft, WHO, 1987). Children identified as "probably autistic" at ages 18-35 months will be assessed and followed on a yearly basis for 2-3 years. Results of observations, parent interviews and tests of communication and intellectual development during these early ages will be compared to results from 1) the same children at ages 3, 4, and 5; and 2) other autistic children who meet standard criteria for autism at ages 3, 4, and 5; 3) mental and chronological age-matched children with Down syndrome or mild developmental delays; and 4) mental age-matched groups of normally developing infants and preschool children. While differences in levels of deficit in each of the three areas (communication, social development, restricted interests and behaviors) are expected to occur across diagnostic groups, standard diagnostic criteria in the areas of communication and interests and behaviors are not expected to discriminate most "probably autistic" children under 3 years as individuals from children in other groups. However, nonverbal communication and social skills, when criteria are appropriate to developmental level and interpersonal contexts are expected to provide clear differentiation. If diagnoses of autism can be made at early ages, baselines for interventions and neurobiological studies can be provided and factors in early development associated with later outcome can then be identified.